I had to vote for Fuko! I just can't get her story arc out of my mind.

Spoiler Alert! Click to show or hide

How everyone started to forget her, even Tomoya and Nagisa after the party they had together

It still sends chills down my spine remembering it all. Just too haunting!
However, if I could have had a second choice, it would be Nagisa........
Darn! All these girls are cute, how am I supposed to pick just one?